I think you would be right about that except for the peculiarities of the cycles. the thorium/u233 one can be done on the fly (I think.) I think i recall that the Thorium U233 cycle is safer environmentally and also from a proliferation perspective because you could make U233 at the burn rate for the reactor so there never was much of it at any one time and the uranium was unsuitable for practically sized fission or fusion bombs.

That's wrong - there would be more than enough U233 to build a bomb. Reactors have hundreds of tons of fuel in them. 1% burnup of thorium in such a reactor would yield some ~1 ton of U233 - enough for a hundred nukes.

The problem (for bomb makers, that is) is that thorium reactors also generate U232, which has very undesirable radiological properties, contaminating that U233.

U233 is a strong gamma-radiator, which makes it too deadly for anyone to isolate into a bomb and carry around.

No, U233 is ok.That's U232. It is a strong gamma-emitter. To be more precise, it has relatively short half-life of 69 years and even though it decays by alpha, all daughters are even more short-lived and you end up having Pb212 and later Tl208, which are beta-active.

No, U233 is ok.That's U232. It is a strong gamma-emitter. To be more precise, it has relatively short half-life of 69 years and even though it decays by alpha, all daughters are even more short-lived and you end up having Pb212 and later Tl208, which are beta-active.

Apologies, now I remember - U232 is the strong gamma-emitter which isn't easily separable from the U233

Anyway, as has been said, the slowness of that part of the decay chain holds everything up.

But thorium can only be used as a fuel in a breeder reactor. If you're going to have a breeder reactor, you can use uranium-238. Uranium-238, like thorium isn't radioactive and can't itself be used as a nuclear reactor fuel. But it can be hit with neutrons to make a radioactive isotope that can be used as nuclear reactor fuel, just like thorium.

I think you mean 'fissile' instead of radioactive; all isotopes of both uranium and thorium are radioactive (though some of them only weakly).

In the early days of the start of the nuclear age there were many types of fuel and reactor designs being considered and tested. Most were flops. In the end it came down to a choice between two fuels; Thorium and Uranium. Both fuels have impressive lists of advantages and disadvantages. Both fuels ran reactors for hundreds of thousands of hours with no problems. In the end it came down to uranium having a single insurmountable advantage over thorium. One needs to consider the context of the times to really understand this but it was a really, really big deal to the United States at the time. The real reason we use uranium over thorium is a result of wartime politics. Cold War-era governments (including ours) backed uranium-based reactors because they produced weapons-grade plutonium —for atomic bombs and icbm warheads. Thorium powered reactors could only produce electricity – no bombs. Every other issue mentioned here is an unimportant aside detail.

Would thorium reactors in space be more "politically correct"? For instance deep space human spacecraft beyond Mars requiring a lot of power that solar cannot give. Or thorium reactors on Mars for night power production or continuous production in event of dust storms. Or large NEP spacecraft that would be smaller than a large SEP spacecraft due to the huge amount of solar panels.

But thorium can only be used as a fuel in a breeder reactor. If you're going to have a breeder reactor, you can use uranium-238. Uranium-238, like thorium isn't radioactive and can't itself be used as a nuclear reactor fuel. But it can be hit with neutrons to make a radioactive isotope that can be used as nuclear reactor fuel, just like thorium.

I think you mean 'fissile' instead of radioactive; all isotopes of both uranium and thorium are radioactive (though some of them only weakly).

True story: Bismuth is now (recently) considered to be radioactive...with a halflife somewhat longer than ( a billion times ) the present age of the universe

Bismuth used to be considered the last stable element in the periodic table. I dunno if this is a promotion or a demotion.

Would thorium reactors in space be more "politically correct"? For instance deep space human spacecraft beyond Mars requiring a lot of power that solar cannot give. Or thorium reactors on Mars for night power production or continuous production in event of dust storms. Or large NEP spacecraft that would be smaller than a large SEP spacecraft due to the huge amount of solar panels.

There's really no such thing as a thorium reactor--thorium is not fissile, though it is fertile. In an appropriate breeder design, thorium (which is more plentiful than uranium and virtually all of it is the "right" isotope) is converted into a fissile isotope of uranium, which is what generates plentiful energy. A compact space-based reactor would more likely be fueled with highly enriched uranium or plutonium, so as not to be complicated by the breeder aspect of the thorium fuel cycle. It may be possible to 'seed' a uranium reactor with thorium which is converted into uranium, but I think that complicates the reactor design and fuel use cycle; it's better to have a reasonably simple design and fuel cycle for space-based nuclear power.

In terrestrial applications, the thorium fuel cycle must necessarily be tied to a significantly complex recycling process to remove certain isotopes and recover the generated U233. There are multiple variations of the thorium fuel cycle and reactor design, and I won't (can't) go into details, especially since many of the details have yet to be designed and verified.

In the early days of the start of the nuclear age there were many types of fuel and reactor designs being considered and tested. Most were flops. In the end it came down to a choice between two fuels; Thorium and Uranium. Both fuels have impressive lists of advantages and disadvantages. Both fuels ran reactors for hundreds of thousands of hours with no problems. In the end it came down to uranium having a single insurmountable advantage over thorium. One needs to consider the context of the times to really understand this but it was a really, really big deal to the United States at the time. The real reason we use uranium over thorium is a result of wartime politics. Cold War-era governments (including ours) backed uranium-based reactors because they produced weapons-grade plutonium —for atomic bombs and icbm warheads. Thorium powered reactors could only produce electricity – no bombs. Every other issue mentioned here is an unimportant aside detail.

False. The plutonium produced in power plants is unsuitable for use in nuclear weapons. Both the United States and Soviet Union produced all their weapons-grade plutonium in special reactors built just for the purpose of making weapons. For the United States, it all came from Savannah River or Hanford. All the plutonium produced by power plants is either buried as waste or reprocessed into more reactor fuel.

The decisions to go with uranium over thorium for power reactors had nothing to do with nuclear weapons.

In the early days of the start of the nuclear age there were many types of fuel and reactor designs being considered and tested. Most were flops. In the end it came down to a choice between two fuels; Thorium and Uranium. Both fuels have impressive lists of advantages and disadvantages. Both fuels ran reactors for hundreds of thousands of hours with no problems. In the end it came down to uranium having a single insurmountable advantage over thorium. One needs to consider the context of the times to really understand this but it was a really, really big deal to the United States at the time. The real reason we use uranium over thorium is a result of wartime politics. Cold War-era governments (including ours) backed uranium-based reactors because they produced weapons-grade plutonium —for atomic bombs and icbm warheads. Thorium powered reactors could only produce electricity – no bombs. Every other issue mentioned here is an unimportant aside detail.

False. The plutonium produced in power plants is unsuitable for use in nuclear weapons. Both the United States and Soviet Union produced all their weapons-grade plutonium in special reactors built just for the purpose of making weapons. For the United States, it all came from Savannah River or Hanford. All the plutonium produced by power plants is either buried as waste or reprocessed into more reactor fuel.

The decisions to go with uranium over thorium for power reactors had nothing to do with nuclear weapons.

The connections linking nuclear power and weapons is more than political or historic. Consider: l FISSIONABLE MATERIALS: It is the same nuclear fuel cycle with its mining of uranium, milling, enrichment and fuel fabrication stages which readies the uranium ore for use in reactors, whether these reactors are used to create plutonium for bombs or generate electricity. In the end, both reactors produce the plutonium. The only difference between them is the concentration of the various isotopes used in the fuel. Each year a typical 1000 mega-watt (MW) commercial power reactor will produce 300 to 500 pounds of plutonium -- enough to build between 25 - 40 Nagasaki-sized atomic bombs.

As Dr. Amory Lovins, director of the Rocky Mountain Institute in Colorado points out, "Every known route to bombs involves either nuclear power or materials and technology which are available, which exist in commerce, as a direct and essential consequence of nuclear power." In order to get plutonium for weapons, one needs a reactor, whether it is a "research" reactor (such as the one which provided India with the fissile material for its first atomic bomb). or a commercial reactor.

Neutrons from the fission of uranium-235 are captured by uranium-238 nuclei to form uranium-239; a beta decay converts a neutron into a proton to form Np-239 (half-life 2.36 days) and another beta decay forms plutonium-239.

All plutonium originates in nuclear reactors and is produced by the capture of extra neutrons by uranium-238 to form U-239, which then undergoes a series of decays to form Pu-239: U-238 + n -> U-239 -> Np-239 -> Pu-239. Some of this plutonium gets consumed by fission before it is removed from the reactor, and some of it gets transmuted to heavier isotopes of plutonium by capturing more neutrons: Pu-239 + n -> Pu-240

Plutonium does not exist naturally to be mined. It is only available as a byproduct of a nuclear fuel cycle that uses uranium as the fuel. There is no nuclear fuel cycle of any kind with thorium that can produce plutonium and THAT is why uranium was chosen as the fuel for nuclear reactors.

In the early days of the start of the nuclear age there were many types of fuel and reactor designs being considered and tested. Most were flops. In the end it came down to a choice between two fuels; Thorium and Uranium. Both fuels have impressive lists of advantages and disadvantages. Both fuels ran reactors for hundreds of thousands of hours with no problems. In the end it came down to uranium having a single insurmountable advantage over thorium. One needs to consider the context of the times to really understand this but it was a really, really big deal to the United States at the time. The real reason we use uranium over thorium is a result of wartime politics. Cold War-era governments (including ours) backed uranium-based reactors because they produced weapons-grade plutonium —for atomic bombs and icbm warheads. Thorium powered reactors could only produce electricity – no bombs. Every other issue mentioned here is an unimportant aside detail.

False. The plutonium produced in power plants is unsuitable for use in nuclear weapons.

Well... if you operate them "normally", yes, the resulting plutonium will be no good for bombs (contaminated by Pu240,241,242). However, if you run power reactor for a short time (a month or even less) and then unload and process the fuel, then you get weapon-grade Pu.

Ironically, that's what effectively happened at Three Mile Island. The accident happened early in the very first fuel campaign of Unit 2, and therefore fuel debris from the TMI accident contained weapon-grade plutonium. After reactor vessel cleanup, It was decided to hand it over for storage to the military.

In the early days of the start of the nuclear age there were many types of fuel and reactor designs being considered and tested. Most were flops. In the end it came down to a choice between two fuels; Thorium and Uranium. Both fuels have impressive lists of advantages and disadvantages. Both fuels ran reactors for hundreds of thousands of hours with no problems. In the end it came down to uranium having a single insurmountable advantage over thorium. One needs to consider the context of the times to really understand this but it was a really, really big deal to the United States at the time. The real reason we use uranium over thorium is a result of wartime politics. Cold War-era governments (including ours) backed uranium-based reactors because they produced weapons-grade plutonium —for atomic bombs and icbm warheads. Thorium powered reactors could only produce electricity – no bombs. Every other issue mentioned here is an unimportant aside detail.

False. The plutonium produced in power plants is unsuitable for use in nuclear weapons.

Well... if you operate them "normally", yes, the resulting plutonium will be no good for bombs (contaminated by Pu240,241,242). However, if you run power reactor for a short time (a month or even less) and then unload and process the fuel, then you get weapon-grade Pu.

Ironically, that's what effectively happened at Three Mile Island. The accident happened early in the very first fuel campaign of Unit 2, and therefore fuel debris from the TMI accident contained weapon-grade plutonium. After reactor vessel cleanup, It was decided to hand it over for storage to the military.

In both the West and the Soviet block, all plutonium ever used in nuclear weapons came from special reactors designed and built explicitly to produce weapons-grade plutonium. None of it ever came from a power reactor -- not even the power reactors designed and owned by the military, such as those in aircraft carriers and submarines.

So the claim that uranium was chosen over thorium for power reactors because it generated plutonium for nuclear weapons is false.

Maybe Iran or North Korean built dual-use reactors that were intended to produce power as a front and secretly to get plutonium too. Maybe even Israel did that. But not the United States. Not Russia. Not China. They had their own, better ways to get plutonium for weapons. They never used or needed to use power reactors to give them plutonium for weapons.

There were many factors going into the decisions to use uranium instead of thorium for power reactors, but for the vast majority of power reactors in the world, those decisions were not made because someone wanted to produce plutonium for nuclear weapons.

Plutonium does not exist naturally to be mined. It is only available as a byproduct of a nuclear fuel cycle that uses uranium as the fuel. There is no nuclear fuel cycle of any kind with thorium that can produce plutonium and THAT is why uranium was chosen as the fuel for nuclear reactors.

I already refuted that in the post you replied to, but you offered nothing that addressed the refutation.

Plutonium does not exist naturally to be mined. It is only available as a byproduct of a nuclear fuel cycle that uses uranium as the fuel. There is no nuclear fuel cycle of any kind with thorium that can produce plutonium and THAT is why uranium was chosen as the fuel for nuclear reactors.

I already refuted that in the post you replied to, but you offered nothing that addressed the refutation.

Please explain how to obtain plutonium without a uranium fuel cycle.

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Chuck - DIRECT co-founderI started my career on the Saturn-V F-1A engine

In the early days of the start of the nuclear age there were many types of fuel and reactor designs being considered and tested. Most were flops. In the end it came down to a choice between two fuels; Thorium and Uranium. Both fuels have impressive lists of advantages and disadvantages. Both fuels ran reactors for hundreds of thousands of hours with no problems. In the end it came down to uranium having a single insurmountable advantage over thorium. One needs to consider the context of the times to really understand this but it was a really, really big deal to the United States at the time. The real reason we use uranium over thorium is a result of wartime politics. Cold War-era governments (including ours) backed uranium-based reactors because they produced weapons-grade plutonium —for atomic bombs and icbm warheads. Thorium powered reactors could only produce electricity – no bombs. Every other issue mentioned here is an unimportant aside detail.

False. The plutonium produced in power plants is unsuitable for use in nuclear weapons.

Well... if you operate them "normally", yes, the resulting plutonium will be no good for bombs (contaminated by Pu240,241,242). However, if you run power reactor for a short time (a month or even less) and then unload and process the fuel, then you get weapon-grade Pu.

Ironically, that's what effectively happened at Three Mile Island. The accident happened early in the very first fuel campaign of Unit 2, and therefore fuel debris from the TMI accident contained weapon-grade plutonium. After reactor vessel cleanup, It was decided to hand it over for storage to the military.

In both the West and the Soviet block, all plutonium ever used in nuclear weapons came from special reactors designed and built explicitly to produce weapons-grade plutonium. None of it ever came from a power reactor -- not even the power reactors designed and owned by the military, such as those in aircraft carriers and submarines.

So the claim that uranium was chosen over thorium for power reactors because it generated plutonium for nuclear weapons is false.

Your conclusion does not follow from the premise - the decision to go with uranium could be influenced by the _possibility_ to quickly make a lot of weapon-grade Pu-239, if necessary - even if the need to actually do that with power reactors never materialized in history.

Moreover, there is testimony from people involved in the nuclear programs of both US and USSR who claim that the decision WAS influenced by these considerations.

Thorium reactors do produce material (U233) that can be used for bombs, it has a critical mass about 50% larger than plutonium 239. It wasn't that thorium doesn't produce fissile material in a reactor, but that plutonium cores could be produced more rapidly than Uranium 233 that made it the preferred core material. It was all limited by the neutron economy (spare neutrons you could generate in the reactor to make new material).

There are three primary isotopes that can be used as fissile materials (materials than can support a nuclear chain reaction). Uranium 233, Uranium 235, and Plutonium 239. You can make bombs or reactors out of any fissile material. Uranium 235 is the only one that occurs naturally because it's the only one with a long enough half life for some of it to still be around since the earth formed. U233 and Pu239 can be made in a reactor by neutron absorption and subsequent beta decay from the fertile materials Thorium 232 and Uranium 238 respectively. Thorium 232 absorbs a neutron and the Thorium 233 double beta decays to U233 with a half life of 27 days. Uranium 238 absorbs a neutron and double beta decays to Pu239 with a half life of 2.4 days, so the process is much quicker. This speed of processing advantage and the smaller critical mass of Pu239 means you can make bombs faster going the Pu239 route.

All the early development of nuclear power came out of defense applications, and that knowledge and experience base had a lot of momentum. The other thing weighing against early thorium power was the necessity of running two different fuel streams instead of one, since you still need to power the reactor with an enriched uranium feed until you can develop the technology to have the breeding ratio high enough for the Pu233 to satisfy the reactor needs. Uranium also is a much simpler fuel stream as it's just once and done, you don't need any complicated processing on highly radioactive material. Until you have a reactor design like a molten salt reactor to simplify handling, even then breeding fuel is more expensive than just digging up more uranium.

For space use breeder reactors of any kind including thorium are not a viable approach. Breeder reactors are an order of magnitude larger in mass, and that just kills the idea right there. For planetary use the trade space changes and it might be a viable option, but not for use in space where minimal mass is so important.

It is true that Thorium is about 3 times more common in the earths crust than Uranium, but a mitigating factor is that Uranium is much more water soluble. This has the effect that it's geologically concentrated and high concentration ores can be found. Thorium on the other hand does not concentrate as easily, but it is extracted as a secondary output from rare earth and other ore streams.

For breeder cycles, an advantage of Th232->U233 over U238->Pu239 is that you can do it with thermal-spectrum neutrons. This makes it possible to build a much more compact reactor, as your fissile inventory can be much smaller and lighter. This advantage becomes practical with continuous fuel reprocessing, which is a factor in the current interest in molten salt reactors.

I suspect that the complexities of reprocessing would offset advantages in a space-borne reactor, but a terrestrial MSR unit (terrestrial on Mars, whatever that's called) for colony power production and industrial process heat may be a good choice.

For propulsion, whatever realizable system that weighs the least and can drive a sufficient amount of your reaction mass out at the highest velocity is what you'd want.

Yes the neutron economy is more favorable with U233 in the thermal range than U235 or Pu239, but even with that advantage a reactor designed to breed it's own fuel will be many times more massive than a burner design, and that's just the reactor core and doesn't include all the inline reprocessing equipment needed to remove the fission products and extract the protactinium 233 produced etc. Fortunately the energy density of nuclear fuel is so high, you can just put sufficient fuel into the core to handle most space based needs without the need to breed fuel.

Kirk is occasionally around, I think he's rather busy these days though.